Add this line after your CSS code:
<body background="imagelink">
Or open your page with notepad and add this in the body class:
background-image: url('imagelink');
It should look like this:
body {
background-image: url('imagelink');
font-family: "Helvetica Neue", HelveticaNeue, helvetica, "Lucida Grande", arial, sans-serif;
color: #898989;
background: #e6e6e6;
}